You won’t find those “cold hard facts” because it is too dependent on the environment. Try to use AGILE in an organization that is not willing nor able, and AGILE will fail you. Agile is a set of principles that I like to think as a specific subset of an overall set of program management principles. If you situation can take advantage of that set over another set, and the environment of correct (or you can work the environment in a way that it will accept the change), then apply the principles and manage the benefits.

Nothing is a silver bullet, and nothing is comprehensive enough that if you do that one set of things your set for success. The successful PM and organization that supports he/her has a collection of tools in the tool bag of which you choise given the challenges in the project you have.